Alternating current limiter



Nov. 8, 1960 Filed May 26, 1955 C. M. DAVIS, JR., ET AL 2 Sheets-Sheet lA. c. SUPPLY CONTROL TRANSFORMER IO f l I4 24 SYNCHRO GENERATOR MAGNETICSERVO AMPLIFIER MOTOR INVENTORS CHARLES M. DAVIS JR. EDWARD T. HOOPERJR.

*RTTORNEYS' c. M. DAVIS, JR., ET AL 2,959,730

ALTERNATING CURRENT LIMITER Nov. 8, 1960 2 Sheets-Sheet 2- Filed May 26,1953 o 1r n\ RADIANS FICA.

'- .006 EDWARD T. HOOPER JR.

O .002 .004 CURRENT lN AMPERS I R \M Ho ATTORNEYS INVENTORS CHARLES M.DAVIS JR.

ALTERNATING CURRENT LIMITER Charles Mitchell Davis, Jr., Washington,D.C., and Edward 'll. Hopper, Jr., Hyattsville, Md., assignors to theUnited. States of America as represented by the Secretary of the NavyFiled May 26, 1953, Ser. No. 357,651

3 Claims. (Cl. 323-89) (Granted under Title 35, US. Code (1952), sec.266) The invention described herein may be manufactured and used by orfor the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

This invention relates to impedance elements and more particularlypertains to non-linear impedance elements.

The present invention relates to an alternating current impedanceelement which presents a low resistive impedance to AC. currents below apredetermined level and a relatively high resistive impedance to AC.currents above that level. The impedance element is particularly adaptedfor use in series with the control transformer of a servo system whereinit is desired to pass low level signals without appreciable attenuation,and wherein it is desired to attenuate higher level A.C. signals inorder to prevent errors from being reflected into other servo systemsconnected to the control transformer and to prevent loading of thecontrol transformer.

An important object of this invention is to provide an impedance elementhaving a 10W impedance to AC. currents below a predetermined level and ahigh impedance to currents above that level.

Another object of this invention is to provide an im pedance element inaccordance with the foregoing object for limiting currents above apredetermined level, which impedance element presents a substantiallyresistive impedance to the circuit. 7

Another object of this invention is to prevent loadin of the controltransformer in a servo system under high level signal conditions and toapply relatively low level signals from the control transformer to theamplifier of the servo system without appreciable attenuation.

A further object of this invention is to provide a current limitingimpedance in the control circuit of a magnetic servo amplifier whichwill not introduce a large time delay into the servo system.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

Fig. 1 is a schematic diagram of a servo system employing the non-linearimpedance limiter;

Fig. 2 is a BH curve illustrating the hysteresis loop of a ferromagneticmaterial;

Fig. 3 is a set of curves illustrating the amplitude and phase relationbetween the applied voltage and exciting currents in the limiter;

Fig. 4 is a set of curves illustrating the voltage current relation forthe non-linear impedance, using different numbers of turns in thewinding.

It has been ascertained that saturable reactors having rectangularhysteresis loops exhibit the characteristics of very non-linearresistors. The action of such an element in a circuit, when operatedbelow saturation, is that of a current limiter possessing low initialimpedance.

The non-linear resistive nature of a winding dis-. posed on a core ofsaturable magnetic material having rates Patent a rectangular hysteresisloop characteristic will best be understood by the following theoreticaldevelopment.

If a sinusoidal voltage is impressed upon a circuit con sisting of aseries inductance L and resistance R, the applied voltage leads thecurrent by an angle 0 where (1) 0=tan- The instantaneous power p can beexpressed as:

(2) p=ie=I V sin wt sin (wt-0) Integrating Equation 2 over a half cycleof the power frequency, the energy dissipated W is:

m m (3) W 4f cos 6 When wL is much greater than R, 0 approaches and Wapproaches zero. The power absorbed by an inductance during a half cycleis therefore returned during the same half cycle. When R is much greaterthan 40L, 0 approaches zero and W approaches and all the power suppliedduring the half cycle is dissipated during the same half cycle. Thus,the smaller that 0 is, the more resistive the circuit appears.

Referring now more specifically to the hysteresis loop illustrated inFig. 2, the energy supplied to the core is expressed by the equation:

m m f From Equation 1, tan 6 is shown to be the energy stored 7 AreaACE'A: cos a per cycle divided by the energy dissipated per cycle, and

therefore Area CEDC' (8) tan Area ACEA also sin 0 (9) tan 0- 6 thereforesin 0 Area CEDC (10 cos 0 n? sin In the case of a closed loop coresuchas toroidal core of a magnetic material having arectangular'hysteresis loop, area ACEA is much greater than area CEDCand tan 0 is small. Therefore tan 0 approaches Zero and consequently 0approaches zero. A saturable reactor comprising a winding on a core ofmagnetic material having rectangular hysteresis loop thus presents asubstantially resistive impedance to an applied voltage when the core isoperated below saturation. The voltage current wave forms for such asaturable reactor are illustrated in Fig. 3, and as is apparent, arenearly in phase.

One such magnetic material having rectangular hysteresis loopcharacteristics is Orthonol, having a composition of approximately 50%Ni 50% Fe and having the properties (11) Area CEDC= max and p20 is thepermeability taken with the B at 20 gauss.

It has been ascertained further that the impedance of such a saturablereactor is very non-linear, as illustrated by the current-voltage curvesshown in Fig. 4. The impedance of the reactor is low for small currentsand increases sharply for currents large enough to place the core on thesteep side of the hysteresis loop but not large enough to saturate thecore. The core, therefore, has the characteristics of a current limiterwhich presents a substantially resistive impedance and possesses theadvantage of a small initial impedance.

In order to achieve the aforementioned current limiting characteristicsit is necessary that the core be operated below saturation. It istherefore necessary that the number of turns and the cross sectionalarea of the core be adjusted such that the flux density produced in thecore by the maximum A.C. voltage to be applied to the winding isinsufficient to drive the core into saturation. This condition can befulfilled by designing the core in accordance with the followingequation:

where N is the number of turns in the winding; A is the cross sectionalarea of the core in square centimeters; E is the full voltage to beapplied to the winding; F is the frequency of the applied voltage and Bis the flux density in the core, the value of which is chosen to be lessthan the residual fiux density Br of the core. Conveniently, B may bemade equal to a value such as .85 Br. 7

As hereinbefore set forth, the limiting action of the impedance elementoccurs when the magnetic intensity in the core approaches the magneticintensity at which the steep portion of the hysteresis loop occurs.However, the area of the hysteresis loop of a core of magnetic materialand consequently the residual fiux density and coercive force varies asa function of the maximum flux density attained during the cycle, theflux density being dependent on the amplitude and the frequency of theapplied voltage. Thus, a relatively high, constant amplitude sinusoidalvoltage applied to the winding on a square loop core will be subjectedto the limiting action during substantially the same portion of eachhalf cycle of the voltage. For rectangular hysteresis loop corematerials, the point at which limiting action occurs is very nearlyequal to the A.C. coercive force of the material at the level of theapplied voltage. It has further been ascertained that the A.C. coerciveforce for all except very low level voltages of rectangular hysteresisloop materials is approximately equal to the A.C. coercive forcemeasured when the core is subjected to a magnetizing force sufficient toproduce the maximum flux density in the core. For a material such asOrthonol, at a voltage which would produce a flux density which is only20% of the maximum flux density, the coercive force is greater than ofthe coercive force measured when maximum fiux density has been attainedin the core.

The impedance element when properly designed may thus be used to limitcurrents produced by a constant amplitude sinusoidal voltage, or to passrelatively low level currents and sharply attenuate currents above apredetermined level. The following equation which expresses the relationbetween the magnetic intensity and the ampere turns per unit length ofthe core may be utilized to design the reactor to obtain the desiredlimiting action.

Where N is the number of turns in the winding; 1 is the mean length ofthe core in centimeters; I is the value of current above which it isdesired to limit, and He is the coercive force of the material. Thevalue of He for convenience may be chosen to be the He measured when thecore has been subjected to an A.C. voltage sufiicient to attain maximumflux density therein, subject to the errors introduced due to the minorhysteresis p eifect produced at lower voltages.

The specific application for which the limiter was designed was toprevent the control transformer in a servo system employing magneticamplifiers from being loaded. Heretofore, a large resistance has beenemployed in series with the control transformer and the control windingsof the magnetic servo amplifier. The impedance of the resistance had tobe much larger than the impedance of the control winding in order toprevent large currents from flowing, and consequently when the sig nalfrom the control transformer is small, the signal applied to themagnetic amplifier control windings would be extremely low.

Reference is now made more specifically to the servo system illustratedin Fig. 1 of the drawings. The servo system comprises an output shaft1%, an input shaft 12, an error detector comprising the synchrogenerator 14 and the control transformer 16 to measure the angulardifference between the input and output shaft and a con trollercomprising the magnetic amplifier 18 and servo motor 20 to regulate themotion of the output shaft in accordance with the magnitude anddirection of the error signal. The synchro generator 14 may be coupledto one shaft such as the input shaft 12 and the control transformercoupled to the other shaft 10. The servo motor 20 is suitably coupled tothe output shaft iti so as to drive the latter in a direction and at arate determined by the error signal from the control transformer.

It has been ascertained that improved servo system response can beachieved by passing the low level control signals to the magneticamplifier 13 without attenuation and by attenuating or limiting higherlevel A.C. signals from the control transformer 16 to prevent theoverloading of the control transformer. In accordance with the presentinvention a saturable reactor limiter comprising a core 22 ofrectangular hysteresis loop material having a winding 24 thereon wasutilized, the win ing 24 being connected in series with controltransformer 16 and the control windings of the magnetic servo amplifier18. As hereinbefore set forth, the saturable reactor limiter using acore material having rectangular hysteresis loop characteristics, whenproperly designed, presents a low impedance to A.C. currents below apredetermined level determined by the design, and presents a highimpedance to AC. currents above that level.

7 Although the impedance of the limiter using rectangular hysteresisloop core material is primarily resistive, the limiter does present asmall inductive impedance which causes the current flowing therethroughto lag the voltage is illustrated by the wave form curves in Fig. 3. Themagnetic amplifier is a carrier type system. The measured phase angle inone application between the current and voltage of the 400 cycle carrierwas 9. This represents a phase shift of less than between the currentand voltage envelopes of the carrier modulated with a 100 radian persecond signal. This delay can be disregarded in a servo system,

A comparison between a two stage magnetic amplifier with no additionalimpedance added to the control circuit; with a 10,000 ohm resistanceadded, and with a limiter added is shown in the table. The values ofvoltage gain with an input signal of V; volt, and the current whichflows with 57 volts out of the signal source for each of the above namedcases is given.

From the above data it can be seen that the limiter when connected inseries with the control transformer and the control winding of themagnetic servo amplifier serves to allow the gain of the magneticamplifier to be increased while limiting the current drawn from thesignal source. This is achieved without introducing an appreciable timedelay'into the servo system.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. In a circuit having a source of voltage, means responsive to saidvoltage connected to said source for applying a load thereto, a currentlimiting non-linear impedance connected in series between said sourceand said means, said impedance comprising a core of magnetic materialhaving rectangular hysteresis loop characteristics, a winding on saidcore, the cross sectional area of the core and the number of turns insaid winding being such that the voltage applied to said winding isinsuflicient to drive the core to saturation, the number of turns insaid winding and the length of said core being such that the magneticintensity in the core is approximately equal to the coercive force ofthe core, when the current through said winding reaches the desiredlimited value.

2. In a circuit, a source of alternating current, means responsive tosaid alternating current connected to said source for applying a load, anon-linear impedance element connected in series between said source andsaid means for limiting the A.C. current flowing therethrough inresponse to the application of an AC. voltage comprising a core ofsaturable magnetic material having rectangular hysteresis loopcharacteristics, a winding on said core, the number of turns in saidwinding and the cross sectional area of said core being proportioned sothat the magnetic flux density produced in the core by the AC. voltageapplied to the winding is less than the satu rated flux density of thecore, the number of turns in said winding and the length of said corebeing proportioned so that the magnetic intensity in the core isapproximately equal to the coercive force of the core, when the currentthrough said winding reaches the desired limited value.

3. In combination with a servo system including a control transformerand a magnetic servo amplifier, said magnetic amplifier having a controlwinding, an alternating current limiter including a core of saturablemagnetic material having rectangular hysteresis loop characteristics, awinding on said core, and means conmeeting said winding in series withthe control winding of said magnetic amplifier and said controltransformer, the cross sectional area of said core and the number ofturns in said winding being such that the full output voltage of saidcontrol transformer applied to the winding of said limiter and saidcontrol winding produces a flux density in said core below the saturatedflux density thereof, the number of turns in said limiter winding andthe length of said core being such that currents in said limiter Windingbelow a predetermined level produce a small change in flux density inthe core and currents above that level produce a large change in fluxdensity in the core.

References Cited in the file of this patent UNITED STATES PATENTS1,920,618 Zierdt Aug. 1, 1933 2,184,371 Thompson Dec. 26, 1939 2,375,609Zuhlke May 8, 1945 2,569,468 Gaugler Oct. 2, 1951 2,661,452 Curry et al.Dec. 1, 1953 2,666,151 Rojchman et al Jan. 12, 1954

